This invention relates to an indirectly heated cathode structure which emits high current density electron beams in electron tubes.
The indirectly heated cathode structure used in the above electron tubes, such as high definition color picture tubes, high-grade pick-up tubes, projection tubes or travelling tubes, usually has a construction in which a supporting sleeve supports a disc-shaped electron emission section. Since, apart from the heater inserted in the sleeve, this cathode supporting sleeve is the part which is exposed to the highest temperature, i.e. 1000.degree. C., it must have a sufficiently high mechanical strength at high temperature Generally, the thicker the supporting sleeve, the higher its mechanical strength. However, a thicker sleeve increases the weight, and it becomes difficult to make the structure compact. Moreover, with a thicker sleeve there would be increased heat loss due to increased heat conduction, and this would result in the disadvantage of requiring greater power for heating. In particular, in the case of an impregnated type cathode structure, a comparatively high operating temperature of 900.degree. C. to 1000.degree. C. (brightness temperature) is typical. Moreover, in the aging process which is carried out prior to use of an electron tube, the sleeve is sometimes heated to approximately 1200.degree. C. Furthermore, electron tubes in which these indirectly heated cathode structures are used are sometimes mounted in satellites, aircraft, ships or automobiles, and therefore more stringent vibration-proofing is required. For these reasons, tantalum (Ta) has been used for the supporting sleeves of conventional impregnated cathode structures.
However, tantalum sleeves often deform at high temperature due to mechanical shocks or vibrations.
It has been suggested in literature that pure niobium, pure tantalum or pure molybdenum might be used as a supporting sleeve (Japanese Patent Application Laid-open No. 54-67757). However, since the strength of niobium at high temperature is lower than that of tantalum, niobium has not been used in practice.